1. Field of the Invention
The present invention relates to an illumination device and a projection device, which are desirable for use in a liquid crystal projector that with a projection lens magnifies and projects images from a liquid crystal display element (i.e., a liquid crystal panel) onto a screen or wall. In particular, the present invention relates to a liquid crystal projector mainly used to display moving pictures on home theater front projectors or rear projection television.
2. Related Background Art
Various types of liquid crystal projectors that illuminate a liquid crystal panel with a luminous flux from a light source and that with a projection lens magnify and project pictures from the liquid crystal panel based on transmitted light or reflected light on a screen or wall have been suggested in the past, and projectors mainly used for displaying moving pictures are being widely sought in order to expand the market size.
Projectors that have been made public so far belong primarily to two representative types: liquid crystal projectors that use liquid crystal display elements, and projection type display elements (hereinafter called “DLP”) based on digital light processing (DLP) that uses a digital mirror device (DMD).
The DLP controls on/off of light by switching at high-speed the angle of a mirror device built on a semiconductor substrate, which controls the proportion of time in the on state within one frame and thereby achieves a gray scale display (i.e., time-division gray scale). On the other hand, such time-division gray scale is not used in liquid crystal display elements other than devices that use ferroelectric liquid crystal. Instead, the liquid crystal display elements utilize a gray scale display method based on analog gray scale, where the value of the voltage applied to the liquid crystal display element is varied according to display picture information in order to display gray scale information.
However, it has become clear from recent research that as long as the two types of devices are driven in the conventional manner, the moving picture high-speed response characteristic that is perceived by humans cannot be obtained from either of them (see “Examination of Moving Picture Quality of Hold, Light Emitting Type Display on an Eight Speed CRT,” Shuichi Ishiguro and Taiichiro Kurita, Technical Report of IEICE EID 96-4, p. 19).
According to the report, when viewing pictures on a liquid crystal display element, the human eye does not sense that the pictures are displayed at high-speed even when the time required for the liquid crystal to respond is speeded up to infinitely close to zero. The reason for this is that the conventional liquid crystal display elements are based on the so-called “hold type display” principle. This is a phenomenon in which display information enters the observer's eyes constantly, and the after image effect of eyes causes the pictures to lose sharpness. A “hold type display” is commonly used in a display method that uses the liquid crystal display elements, as well as in a method of displaying gray scale based on time-division in DLP.
As long as this display method is used, the moving picture display performance cannot improve significantly; the research result concludes that effective methods for improving moving picture quality, to the extent that humans sense that a moving picture is displayed at a high-speed, include a method of using a shutter to reduce the time numerical aperture to 50% or less, and a display method employing double speed or higher.
Of the two methods for improving the moving picture quality, the display method employing double speed or higher entails burdening drive circuits, e.g., requiring complicated picture processing such as picture interpolation and requiring even higher speed density such as quad speed (4×) or five speed (5×) in order to obtain a moving picture quality equivalent to that of CRT, and is therefore not a realistic method for achieving true high-speed display performance.
In contrast, the method of reducing the time numerical aperture by using a shutter (a so-called “non-hold type display method”) can be realized fairly easily, since the original picture information can be used unaltered.
The non-hold type display method developed for direct view type liquid crystal display elements can be divided into two methods. One is a method of using high-speed liquid crystal to switch the light on/off, which can be achieved by using ferroelectric liquid crystal or optically compensated bend (OCB) mode.
The other method is to turn a backlight on and off. Since this does not require the response speed of the liquid crystal to be especially high-speed, this method can be applied to virtually all liquid crystal modes.
Since the screen size displayed by a projector is much larger than that of a direct view type liquid crystal display element, a moving picture quality in the projection type display is sought to be much higher than that of the direct view type liquid crystal display element in the future. The reason for this is that when the viewing distance remains the same, as the screen size becomes larger, the angle between the human eyes and both ends of the screen becomes larger. As a result, when the same moving picture is displayed, the angular speed with which pictures move becomes larger as the screen becomes larger. However, when the non-hold display method is used on a liquid crystal projector or DLP, the following problems arise:
First, when the method of using high-speed liquid crystal to switch the light on/off is utilized, the utilization rate of light is determined by the proportion of the on time, while the utilization rate of light declines considerably according to the proportion of the off time. This can be dealt with fairly easily with direct view type, such as by increasing the luminance of the backlight, but increasing the luminance of the light source lamp is difficult with projectors.
Consequently, efforts are underway on projectors to realize a display that is even slightly brighter than currently available through a combination of various approaches, such as optics and a display element, in order to increase the utilization rate of light. Due to such circumstances, creating off periods through liquid crystal switching is not a desirable method for projectors, since this would significantly decrease the utilization rate of light.
In the meantime, the method of turning the backlight on and off is a method that can be used only on direct view type liquid crystal elements that utilize a fluorescent tube; turning a light source on and off is impossible with halogen lamps that are generally used on projectors.
Another possible method is to place a shutter wheel in front of a light source and rotate the shutter wheel to create light and darkness from the light source; however, this is equivalent to turning the light on and off through liquid crystal switching and would therefore significantly reduce the utilization rate of light.
It has been pointed out that it is difficult to obtain a sharp display of moving pictures, as well as sufficient dynamic range due to lack of contrast in a display element, with conventional projectors. In other words, conventional projectors lack moving picture quality and dynamic range, which are essential factors for achieving vivid visual expression.